Thrug spawns in Thrug's Torture Chamber ( -25, -10, -13 ) /waypoint -25, -10, -13 but you cannot unlock the door to the room until you retrieve the key in one of the locked jail cells.
Click on the dying Jailor Qulkor, who tells you that he has locked Thrug away and thrown the key into a cell.
He then tells you how to find the cell. For example, "...from here... 2nd right... 2nd left... 1st left... 1st right" or "...from here... 1st left... 2nd left... 2nd right... 1st right" and so on
If you do not unlock the cells in the given order, you die instantly.
The key is in the last cell that you unlock. It's often somewhat concealed and small, but has a sparkle effect.
Thrug casts a very high-damage spell, Bloody Mess that is usually an instant kill. The spell has a very small radius, so joust out!
Thrug can only be damaged while he is moving, so you'll want to run back and forth down the hall. A slow spell helps a lot here.
After Thrug is dead, the iron maiden in his room becomes active. You can open it, run down the hall, climb the rope, and you'll appear outside on the trebuchet near The Disabled Arm of Innoruuk. You can jump back down the opening to return to the torture chamber.